Ethernet network devices are typically tested following manufacture to verify proper operation. For example, an Ethernet network device may be required to have less than a minimum number of cyclic redundancy check (CRC) errors, symbol errors, and/or false carriers when a known signal is transmitted to the device. Automatic testing equipment (ATE) systems may be employed to perform network device testing. The ATE system typically includes another network device and/or a signal generator that transmits data to and receives data from the Ethernet network device.
Referring to FIG. 1, an ATE system 10 transmits/receives data to/from a device under test (DUT) 12. In an exemplary embodiment, the ATE system 10 includes an arbitrary waveform generator (AWG) 14 and the DUT 12 is an Ethernet network device 12. A data communications medium 16 connects the AWG 14 to the Ethernet network device 12. For example, the data communications medium may be cable including two or four pairs of twisted wire or another suitable medium. While the ATE system 10 tests the Ethernet network device 12, the AWG 14 is capable of emulating different lengths of the data communications medium 16. For example, the AWG 14 may store programs for different data communications medium 16 lengths. A first program may test the Ethernet network device 12 with an emulated 50 m data communications medium 16, and a second program may test the Ethernet network device 12 with an emulated 100 m data communications medium 16. Still other cable lengths may be used. This enables the ATE system 10 to test the Ethernet network device 12 over a wide range of conditions.
The Ethernet network device 12 includes a physical layer device 18 that controls access to the data communications medium 16. The physical layer device 18 includes devices that are specific to a desired communications architecture. For example, the physical layer device 18 in FIG. 1 is compliant with IEEE 100BASE-TX. The physical layer device 18 includes a physical coding sublayer (PCS) device 20, which acts as an interface between a medium independent interface (MII) and a physical medium attachment (PMA). The PCS device 20 typically includes an encoder/decoder that formats data. The PCS device 20 also typically includes a data scrambler that performs line balancing and ensures that there is sufficient transition density in data that is transmitted to the AWG 14. For example, the data scrambler may reduce electromagnetic interference by randomizing the transmitted data.
The data scrambler implements one or more scrambling cycles that are defined by polynomials. Each scrambling cycle includes a limited number of bit combinations. The recurring bit combinations are used to scramble data that is transmitted by the Ethernet network device 12. For example, the data scrambler may include a master scrambling cycle and a slave scrambling cycle. The master scrambling cycle is used when the Ethernet network device 12 initiates communications with the AWG 14. The slave scrambling cycle is used when the AWG 14 initiates communications with the Ethernet network device 12.
During a testing operation, the AWG 14 transmits data packets to the Ethernet network device 12 and determines whether the packets are successfully received by the Ethernet network device 12. Corrective action may be taken when the Ethernet network device 12 has a minimum number of packet errors. The AWG 14 simultaneously receives data from the Ethernet network device 12 to simulate two-way communications. For example, the AWG 14 may transmit a stored testing sequence that includes idle symbols to emulate a desired length of the data communications medium 16.
Since the Ethernet network device 12 may not initially be prepared to receive data, the AWG 14 seamlessly loops the testing sequence. Therefore, the Ethernet network device 12 begins receiving the testing sequence once the Ethernet network device 12 is prepared to receive data. The length of the testing sequence depends on the length of the scrambling cycle that is employed by the data scrambler of the Ethernet network device 12. The length of a scrambling cycle that is used by a scrambler in a 100BASE-TX PCS device 20 is relatively short. Therefore, the testing sequence is short enough to loop and store in the AWG 14.
Referring now to FIG. 2, conventional Ethernet network devices alternatively or additionally include Gigabit Ethernet communications architectures. A physical layer device 28 of an Ethernet network device 30 includes an PCS device 32 that is compliant with IEEE 1000BASE-T. The PCS device 32 includes a data scrambler 34 with a master scrambling cycle 36 and a slave scrambling cycle 38. The master and slave scrambling cycles 36 and 38, respectively, are set according to IEEE standards. For example, the IEEE 1000BASE-T master scrambling cycle 36 implements the polynomial x33+x13+1. The IEEE 1000BASE-T slave scrambling cycle 38 implements the polynomial x33+x20+1.
Referring now to FIGS. 3A and 3B, the master and slave scrambling cycles 36 and 38, respectively, include 33-bit outputs. For example, the data scrambler 34 may utilize linear feedback shift registers (LFSRs) with side-stream scrambler (SSR) functions to generate scrambler outputs. In FIG. 3A, the master scrambling cycle 36 is implemented by an LFSR 46. After each cycle time, bits are shifted one bit position to the right. An exclusive-OR (XOR) gate 48 performs an XOR operation on previous bits from the 12th and 32nd bit positions 50-1 and 50-2, respectively. An output of the XOR gate 48 replaces the 0th bit position 50-3 for the next cycle. The output of the data scrambler 34 is a 33-bit number that includes current values of the 33 bit positions.
In FIG. 3B, the slave scrambling cycle 38 is also implemented by an LFSR 52. However, an XOR gate 54 performs an XOR operation on previous bits from the 19th and 32nd bit positions 56-1 and 56-2, respectively. The output of the XOR gate 54 replaces the 0th bit position 56-3 for the next cycle. Since an ATE and DUT simultaneously transmit data to each other during testing, it is desirable for the master and slave scrambling cycles 36 and 38, respectively, to be different. This keeps the signals uncorrelated.
Both the master and slave scrambling cycles 36 and 38, respectively, repeat after 233−1=8,589,934,591 cycles. The reason that the patterns do not repeat after 8,589,934,592 cycles is because a pattern of all 0's is not allowed according to IEEE standards. For example, when symbols are separated by 8 nanoseconds, the bit pattern repeats after approximately 68.72 seconds. An AWG is not typically able to store and loop a testing pattern of this size. Therefore, due to the length of the scrambling cycles, an AWG is not typically used to test an Ethernet network device with a PCS device 32 that is compliant with IEEE 1000BASE-T.
Referring now to FIG. 4, in one conventional method, an ATE system 64 includes a golden device 66 and is used to test an Ethernet network device 68. A golden device 66 is a device that is known to operate properly before a testing operation so that failures occurring during a testing operation are automatically attributable to the device under test 68. The Ethernet network device 68 includes a physical layer device 70 with a PCS device 72 that is compliant with 1000BASE-T. The golden device 66 may scale down a data rate of the Ethernet network device 68 and scale up a data rate of the ATE system 64 so that the rates match. For example, the golden device 66 may employ multiplexing/demultiplexing hardware. A data communications medium 74 connects the golden device 66 and the Ethernet network device 68. However, unlike an AWG, the golden device 66 does not emulate different lengths of the data communications medium 74. Therefore, data communications media of different lengths are required to simulate data communications across network connections of different lengths. This is both time consuming and expensive.
Typically, the Ethernet network device 68 is only tested with a data communications medium 74 having a length that is equal to a maximum distance that the Ethernet network device 68 reliably transmits/receives data. For example, a 100 m data communications medium 74 is typically used to test an Ethernet network device 68 with a PCS device 72 that is compliant with 1000BASE-T. This prevents a user from determining differences in analog sensitivity between data communications media of different lengths and significantly limits the range of conditions under which the Ethernet network device 68 may be tested.